Patent application title: Method and Device For Producing an Image

Abstract:

The invention relates to a method and to a device (1) for producing an
image of an object (5) by means of a particle beam. According to the
method and in the device (1), the particle beam is scanned by the object
(5). The aim of the invention is to provide a method and a device for
producing an image of an object (5) by means of a particle beam that can
be used with a cold field emitter (2) in such a manner that a good image
quality is constantly ensured. Said aim is achieved by virtue of the fact
that, according to the invention, when a radiation parameter is altered,
the object (5) is rescanned preferably with the corrected parameter. The
inventive device (1) comprises the corresponding means (4, 6, 7)
therefor.

Claims:

1. A method for producing an image of an object using a particle beam that
is scanned over the object, comprising:determining a parameter that is
assigned to the particle beam;determining whether a change has occurred
in the parameter;determining a location on the object at which the change
occurred in the parameter;returning the particle beam to the location;
andrescanning the particle beam over the object from the location onward.

2. The method as recited in claim 1, wherein the parameter is changed to a
setpoint value.

3. The method as recited in claim 1, wherein the parameter is integrated
over a specified period of time, and wherein the method further
comprises:determining whether a change has occurred in the parameter.

4. The method as recited in claim 1, wherein the particle beam is scanned
over the object from a first point of a specified pattern to a second
point of the specified pattern, and wherein, if is determined that a
change has occurred in the parameter, the particle beam is not returned
during the scanning process until the particle beam has reached the
second point of the specified pattern.

5. The method as recited in claim 4, wherein at least one of: a row and a
column is used as the specified pattern.

6. The method as recited in claim 1, wherein a current of the particle
beam is determined as the parameter.

7. The method as recited in claim 1, wherein the particle beam is scanned
over the object using a scanner which is controlled by an evaluation
unit.

8. The method as recited in claim 7, wherein at least one of: the
parameter and the location is determined via the evaluation unit.

9. The method as recited in claim 1, wherein the particle beam is an
electron beam, and wherein the electron beam is scanned over the object.

10. The method as recited in claim 1, wherein the particle beam is
produced using at least one of: a cold field emitter and a deposited
field emitter.

11. (canceled)

12. The method as recited in claim 1, wherein a threshold value of the
change in the parameter is settable.

13. The method as recited in claim 1, wherein the particle beam is guided
to at least one raster point, at which an integrating measurement is
carried out, and wherein the particle beam is returned to the raster
point if the raster point is the location at which the change occurred in
the parameter.

14. The method as recited in claim 1, wherein, when a change has occurred
in the parameter, at least one of: an image brightness and an image
contrast is adjusted.

15. (canceled)

16. A device for producing an image of an object using a particle beam
that is scanned over the object, comprising:at least one particle beam
generator for producing a particle beam;at least one scanner for scanning
the particle beam over the object;at least one parameter determining
device for determining a parameter that is assigned to the particle
beam;at least one change determining device for determining that a change
has occurred in the parameter;at least one location determining device
for determining a location on the object at which the change occurred in
the parameter; andat least one returning device for returning the
particle beam to the location.

17. The device as recited in claim 16, wherein a controller is provided
for changing the parameter to a setpoint value.

18. The device as recited in claim 16, wherein the returning device for
returning the particle beam is integrated in the scanner.

19. The device as recited in claim 16, wherein the returning device for
returning the particle beam and the scanner are identical.

20. The device as recited in claim 16, wherein the parameter determining
device for determining the parameter assigned to the particle beam
determines the particle beam current.

21. The device as recited in claim 16, wherein the parameter determining
device for determining the parameter assigned to the particle beam is at
least one of: located between the particle beam generator and the object
and integrated in the particle beam generator.

22. The device as recited in claim 16, wherein the change determining
device for determining that a change has occurred in the parameter is
accommodated in the location determining device for determining the
location.

23. The device as recited in claim 16, wherein the change determining
device for determining that a change has occurred in the parameter and
the location determining device for determining the location are
identical.

24. The device as recited in claim 16, wherein at least two of: the
parameter determining device for determining the parameter, the change
determining device for determining that a change has occurred in the
parameter, the location determining device for determining the location
on the object, and the returning device for returning the particle beam
are connected to each other.

25. The device as recited in claim 17, wherein the controller for changing
the parameter is connected to the particle beam generator.

26. The device as recited in claim 16, wherein at least one of: the
parameter determining device for determining the parameter, the change
determining device for determining that a change has occurred in the
parameter, and the location determining device for determining the
location on the object is located in an evaluation unit.

27. The device as recited in claim 16, wherein the particle beam generator
is at least one of: an electron source, a cold field emitter, and a
miniaturized cold field emitter.

28. (canceled)

29. The device as recited in claim 16, wherein the device is an electron
beam device.

30. The device as recited in claim 16, further comprising:a setting device
for setting a threshold value with regard to the change in the parameter.

31. The device as recited in claim 16, further comprising:a measuring
device for performing an integrating measurement.

32. The device as recited in claim 16, further comprising:an adjustment
device for adjusting at least one of: an image brightness and an image
contrast.

33. An electron beam device for producing an image of an object using an
electron beam that is scanned over the object, comprising:at least one
electron beam generator for producing an electron beam;at least one
objective lens for focusing the electron beam on the object;at least one
scanner for scanning the electron beam over the object;at least one
parameter determining device for determining a parameter that is assigned
to the electron beam;at least one change determining device for
determining that a change has occurred in the parameter;at least one
location determining device for determining a location on the object at
which the change occurred in the parameter; andat least one returning
device for returning the electron beam to the location.

34. The electron beam device as recited in claim 33, further comprising:a
controller for changing the parameter to a setpoint value.

Description:

[0001]The present invention relates to a method and a device for producing
an image of an object using a particle beam; in the method or the device,
the particle beam is scanned over the object.

[0002]The aforementioned device is a particle beam system. It may be
designed, e.g., as an electron beam system or an ion beam system. The use
of an electron beam system in particular is already very widespread. An
electron beam system is used, e.g., to manufacture nanostructures. An
example of an electron beam system is an electron beam microscope, which
has been known for a long time, and using which an image of an object may
be created. The two known types of electron beam microscopes are the
scanning electron microscope and the transmission electron microscope.
The scanning electron microscope is used to create high-resolution images
of semiconductor structures, biological and mineralogical samples, and
other samples.

[0003]An electron beam system (also referred to as an electron beam
device) such as the scanning electron microscope includes a particle beam
generator in the form of an electron beam generator for producing an
electron beam, an objective lens for focusing the electron beam on the
object, and at least one detector for detecting electrons that are
scattered on the object or that are emitted from the object. The electron
beam produced by the particle beam generator is focused via the objective
lens on the object to be investigated. Using a deflection device, the
electron beam is guided in a scanning pattern over the surface of the
object to be investigated. The electron beam is therefore scanned over
the surface of the object. The electrons of the electron beam interact
with the object. As a result of the interaction, electrons, in
particular, are emitted from the object surface ("secondary electrons"),
or electrons from the electron beam are reflected back ("backscattering
electrons"). Secondary electrons and backscattering electrons may be
detected using the detector. The detector detects a detector signal
depending on the detected secondary electrons and backscattering
electrons. The detector signal is used to create an image.

[0004]Thermal field emitters or cold field emitters, for example, may be
used as the particle beam generators (which are also referred to below as
a particle beam source). The use of field emitters of this type has been
known for a long time. Cold field emitters that are miniaturized have
been used for various applications over the past few years. DE 103 02 794
A1, for example, describes miniaturized electron beam systems having cold
field emitters, which are also miniaturized, and which are manufactured
using electron beam-induced deposition. In addition, EP 1 186 079 B1
describes a miniaturized electron beam source that operates in the
terahertz range.

[0005]Cold field emitters have high beam directionality and a low beam
energy width. They are therefore particularly well-suited for attaining
very small electron beam diameters, thereby making it possible to attain
a high resolution. They have the disadvantage, however, that their
electron emission current fluctuates over time. The fluctuations over
time in the electron emission current become apparent as disturbing light
or dark strips in the line grid image when an object is imaged, e.g., in
a scanning electron microscope. This is an undesired phenomenon.

[0006]The object of the present invention, therefore, is to provide a
method and a device for producing an image of an object using a particle
beam, the method and device being used with a cold field emitter in a
manner such that good image quality is always ensured.

[0007]The object is attained according to the present invention with a
method having the features of Claim 1. A device according to the present
invention is characterized by the features of Claim 16. Further features
of the present invention result from the further claims and the
description that follows.

[0008]The method according to the present invention for producing an image
of an object using a particle beam produces the image by scanning the
particle beam over the object. To improve the image quality, a parameter
associated with the particle beam is determined. It is also determined if
the parameter changes. If a change has occurred in the parameter, the
location on the object at which the change occurred in the parameter is
determined. The particle beam is then returned to this location. The
particle beam is then rescanned over the object from this location
onward.

[0009]The present invention is based on the finding that the property of a
particle beam generator, in particular a cold field emitter, to only show
abrupt changes in the emission current and to show a constant emission
current between these abrupt changes may be used to advantage to create
images. With particle beam generators, the emission current typically
changes abruptly within a very short period of time, usually less than a
microsecond. After an abrupt change takes place, the emission current
remains stable for a certain period of time before it "jumps" again. The
present invention is based on responding to a determination that a change
has occurred in the emission current, and on taking steps to improve the
image quality. If it is determined that the emission current of the
particle beam generator has changed, the location at which the change in
emission current took place is determined immediately. The particle beam
is returned to this location, and the particle beam is rescanned over the
object from this location onward. The faulty image information that was
obtained from scanned image points of the object is deleted and replaced
with the information that is obtained in the rescanning. The rescanning
procedure has a negligible impact on the time required to capture an
image.

[0010]In the method according to the present invention, the parameter is
preferably changed to a setpoint value after it is determined that a
change has occurred in the parameter. For example, when it is determined
that a change has occurred in an emission current of an emitter, the
emission current is changed to a setpoint value, thereby ensuring that
the quality of the imaging is always sufficiently high, and, in
particular, that the brightness of the image is of good quality and is
sufficient.

[0011]The location at which the change occurred in the parameter is
determined, e.g., using a calculation of time, preferably with
consideration for the point in time at which the change was detected, the
speed of the scanning by the particle beam, and/or signal transit times.

[0012]Preferably, the magnitude of the change in the parameter that
results in the particle beam being returned may be set. A threshold value
may thus be set. This ensures that every minimal fluctuation of the
parameter does not result in the particle beam being returned, and only
that significant changes result in the particle beam being returned.

[0013]In a particular embodiment of the method according to the present
invention, the parameter is integrated over a specified period of time,
and only then is it determined whether a change has occurred in the
parameter. This procedure is advantageous, in particular, when observing
low-intensity parameters. To obtain an adequate signal with a high
signal-to-noise ratio, the parameter is integrated over a certain period
of time.

[0014]In a further exemplary embodiment of the method according to the
present invention, the particle beam is scanned over the object from a
first point of a specified pattern to a second point of the specified
pattern. If it is determined that a change has occurred in the parameter,
the particle beam is not returned to the location where the change
occurred in the parameter until the particle beam has reached the second
point of the specified pattern during the scanning process. In this
exemplary embodiment, therefore, the particle beam is not rescanned over
the object until the specified pattern has been scanned between two
points. The first point is preferably the beginning of the specified
pattern, and the second point is considered to be the end of the
specified pattern.

[0015]The specified pattern is, e.g., a row (image line) or a column
(image column). The correction is carried out after this row or column
has been scanned. As an alternative, it is provided that several
consecutive rows or columns are used as the pattern. The present
invention is not limited to a row-based or column-based pattern, however.
Instead, it is possible to use any pattern that may be used for scanning,
i.e., recording data from point to point on an object.

[0016]In a particular embodiment of the method according to the present
invention, a current of the particle beam (also referred to as the
particle beam current or particle current) is determined as the
parameter. For example, the particle beam current is the emission current
of electrons from a cold field emitter.

[0017]In a further embodiment of the method according to the present
invention, the particle beam is scanned over the object using a scanning
means, which is controlled by an evaluation unit. Preferably, the
parameter, and--as an alternative or in addition thereto--the location at
which the change occurred in the parameter are determined using the
evaluation unit.

[0018]In a preferred embodiment of the method according to the present
invention, the particle beam is designed as an electron beam, which is
scanned over the object in order to create an image.

[0019]It is preferably provided that the particle beam is produced using a
cold field emitter. For this purpose, it is possible in particular to use
a miniaturized cold field emitter, since this type of field emitter
requires smaller voltage changes in order to control the particle beam
(electron beam) than does a macroscopic emitter. For example, the
particle beam is produced using a deposited cold field emitter, which is
understood to be a miniaturized cold field emitter that was manufactured,
e.g., using electron beam-induced deposition.

[0020]In a further embodiment of the method according to the present
invention, the particle beam is guided to at least one raster point,
where an integrating measurement is carried out. For example, during
scanning, a particle beam is guided over the object to the raster point,
where a measurement is performed repeatedly using the particle beam, and
the measured values obtained in this manner are integrated in order to
calculate the mean (pixel integration). The integrating measurement may
also be carried out for all raster points located in one raster line
(line integration). As an alternative, the integrating measurement may
also be carried out for several raster lines, each of which includes
raster points, and which are combined to form a single unit (frame
integration). If it is determined in the pixel integration that a change
has occurred in the parameter at the aforementioned raster point, the
particle beam is returned to this raster point. If it is determined in
the line integration that the change occurred in the parameter at one of
the raster points in the raster line, the particle beam is returned to
this raster point or another raster point in the raster line that is
located before the raster point at which the change occurred in the
parameter (e.g., the first raster point in the raster line, i.e., the
beginning of the raster line). The same applies to the frame integration,
in which case it is also provided that the particle beam is returned to
the raster point at which the change occurred in the parameter, or it is
returned to another raster point in the frame located before this raster
point (e.g., the beginning of one of the raster lines). In all of the
exemplary embodiments, the rescanning is started once the particle beam
has returned.

[0021]In a further embodiment of the method according to the present
invention, it is provided that the brightness and contrast of the
generated image are adjusted. The brightness and contrast are adjusted
automatically, for example. If a change occurs in the parameter of the
particle beam, the brightness is adjusted. Abrupt changes in brightness
are largely prevented by using the method described of returning the
particle beam to the location of the change and repeating the scanning
procedure.

[0022]The method according to the present invention is preferably used in
a particle beam device, e.g., an electron beam device, which is described
in greater detail below.

[0023]The present invention also relates to a device for producing an
image of an object using a particle beam, using which the particle beam
is scanned over the object. The device according to the present invention
is designed, in particular, to carry out a method having at least one of
the aforementioned features or combinations of features. The device
includes at least one particle beam generator for producing a particle
beam, at least one scanning means for scanning the particle beam over the
object, at least one means for determining a parameter assigned to the
particle beam, at least one means for determining that a change has
occurred in the parameter, at least one means for determining a location
on the object at which the change has occurred in the parameter, and at
least one means for returning the particle beam to this location.

[0024]In the device according to the present invention, the particle beam
that is scanned over the object using the scanning means is produced
using the particle beam generator. It is possible to determine the
location at which the parameter changed by using the means for
determining the parameter associated with the particle beam, and by using
the means for determining whether a change has occurred in the parameter.
This is carried out, e.g., using the calculation of time described above.
The particle beam is then returned to this location and the rescanning of
the object is started, as described above.

[0025]In the device according to the present invention, it is preferably
provided that a means is provided for changing the parameter to a
setpoint value, the means being used to change the parameter as described
above.

[0026]The device according to the present invention preferably also
includes a means for setting a threshold value for the parameter change.
In this manner, it is ensured that--as described above--every minimal
change in the parameter does not result in the particle beam being
returned.

[0027]In one embodiment of the device according to the present invention,
the means for returning the particle beam is situated in the scanning
means. The means for returning the particle beam and the scanning means
are preferably identical in design. They therefore form a single unit.

[0028]It is also preferably provided that the means for determining the
parameter associated with the particle beam is designed as a means for
determining the particle beam current. The emission current of a cold
field emitter that is used as the parameter is therefore measured, for
example.

[0029]The means for determining the parameter associated with the particle
beam is located between the particle beam generator and the object. As an
alternative, it is provided that the means for determining the parameter
assigned to the particle beam is integrated in the particle beam
generator. For example, the means for determining the parameter is
located in an electron beam column between a cold field emitter and an
object, the emission current of the cold field emitter being used as the
parameter in this case.

[0030]In a further embodiment of the device according to the present
invention, the means for determining that a change has occurred in the
parameter is situated in the means for determining the location. It is
preferably provided that the means for determining a change that occurred
in the parameter, and the means for determining the location are designed
to be identical, i.e., they are designed as one unit.

[0031]In a preferred embodiment of the device according to the present
invention, the means for determining the parameter, and/or the means for
determining that a change has occurred in the parameter, and/or the means
for determining the location on the object, and/or the means for
returning the particle beam is/are connected with each other. The means
for determining the parameter, the means for determining that a change
has occurred in the parameter, and/or the means for determining the
location on the object is/are preferably accommodated in an evaluation
unit.

[0032]It is also preferably provided that the means for changing the
parameter is connected to the particle beam generator.

[0033]In a further embodiment of the present invention, the device is
provided with a means for performing an integrating measurement, as
described above. It is also preferably provided that the device includes
a means for adjusting the brightness and/or contrast of the image that
was created.

[0034]In a further embodiment of the present invention, the particle beam
generator is designed as an electron beam source. The device is therefore
an electron beam device, e.g., of the type described below.

[0035]The present invention also relates to an electron beam device, in
particular a scanning electron microscope, to create an image of an
object using an electron beam, the electron beam being scanned over the
object. The electron beam device according to the present invention
includes at least one electron beam source for producing an electron
beam, at least one objective lens for focusing the electron beam onto the
object, at least one scanning means for scanning the electron beam over
the object, at least one means for determining a parameter assigned to
the electron beam, at least one means for determining that a change has
occurred in the parameter, at least one means for determining a location
on the object at which the change occurred in the parameter, and at least
one means for returning the electron beam to this location.

[0036]It is also provided to locate a means for changing the parameter to
a setpoint value on the electron beam device according to the present
invention.

[0037]The present invention is explained below in greater detail with
reference to exemplary embodiments using figures.

[0038]FIG. 1 shows a schematic depiction of a particle beam device;

[0039]FIG. 2 shows a schematic depiction of an electron beam device
designed as a scanning electron microscope;

[0040]FIGS. 3a through 3c show a schematic depiction of an object having
raster image points;

[0041]FIGS. 4a through 4d show a further schematic depiction of an object
having raster image points; and

[0042]FIGS. 5a through 5c show a further schematic depiction of an object
having raster image points.

[0043]The present invention will now be explained in general with
reference to a particle beam device 1. FIG. 1 shows the schematic
depiction of particle beam device 1. Particle beam device 1 includes a
beam source 2 (beam generator), using which a particle beam is generated.
The particle beam is introduced into a particle beam column (not
depicted), and it is focused onto an object 5 using an objective lens 3.
A scanning means 4 is provided in order to scan the particle beam over
object 5. It is expressly pointed out that the relative placement of
objective lens 3 and scanning means 4 is merely a schematic depiction.
The actual placement of objective lens 3 and scanning means 4 relative to
each other may be entirely different from that which is shown.

[0044]The particle beam scanned over object 5 using scanning means 4
interacts with object 5 and generates interaction particles, which are
detected via a detector (not depicted) and are used to create an image.

[0045]Particle beam device 1 also includes a particle current measuring
device 6, which is located between particle generator 2 and objective
lens 3. As an alternative, it is provided that the particle current
measuring device is integrated in beam generator 2. Particle current
measuring device 6 is connected via a line 8 to an evaluation unit 7. In
addition, evaluation unit 7 is connected via a line 9 to scanning means
4. It is also possible to use evaluation unit 7 to set a threshold value
for the change in the particle current. If the change in the particle
current exceeds this threshold value, this causes the particle beam to be
returned, as described below.

[0046]The particle current of the particle beam generated by beam
generator 2 is measured using particle current measuring device 6. The
particle current and its value determined in this manner are forwarded
via line 8 to evaluation unit 7. Evaluation unit 7 is provided with a
control program in such a way that evaluation unit 7 may determine
whether a value of the particle current that was just measured has
changed compared to a previously measured value of the particle current.
For this purpose, evaluation unit 7 is provided with a microprocessor
(not depicted) and memory components (not depicted), in which, in
particular, the control program and values of the particle current are
stored, or are continually saved. As soon as evaluation unit 7 has
determined that a change has occurred in the particle current (the
threshold value has been exceeded), the location of object 5 at which the
particle beam was located when the particle current changed is determined
via a time calculation. The particle beam is then returned to this
location. For this purpose, evaluation unit 7 sends appropriate control
signals via line 9 to scanning means 4, which returns the particle beam
to the location. The scanning of object 5 is then repeated starting at
this location, and it is continued from this location onward. The faulty
image information that was obtained from scanned image points of object 5
is deleted and replaced with the information that is obtained in the
rescanning procedure.

[0047]FIG. 2 shows a particular embodiment of the device according to the
present invention, which is designed as an electron beam device in the
form of a scanning electron microscope 1. It includes a beam source 2 in
the form of a cold field emitter, an extraction electrode 10, and an
anode 11, which is also one end of a beam guide tube 12 of scanning
electron microscope 1. Electrons that exit beam source 2 are accelerated
to anode potential due to a potential difference between beam source 2
and anode 11.

[0048]Beam guide tube 12 passes through a bore formed through pole shoes
16 of a magnetic lens acting as objective 3. Coils 17 are located in pole
shoes 16. An electrostatic retardation device is connected downstream
from beam guide tube 12. The electrostatic retardation device is composed
of an electrode 19 and a tube electrode 18 formed on the end of beam
guide tube 12 situated opposite an object 5. Tube electrode 18 and beam
guide tube 12 are therefore at anode potential, while electrode 19 and
object 5 are at a potential that is lower than the anode potential. In
this manner, the electrons in the electron beam exiting beam source 2 may
be slowed to a desired low energy that is required to investigate object
5. A scanning means 4 is also provided, via which the electron beam may
be deflected and scanned over object 5.

[0049]To detect secondary electrons or backscattered electrons produced
via the interaction of the electron beam generated by beam source 2 with
object 5, a detector system having a first detector 14 and a second
detector 15 is located in beam guide tube 12. Detector 14 is located in
beam guide tube 12 in such a way that it is movable along optical axis 20
using an adjusting device 13. First detector 14 and second detector 15
each include an annular detection surface. It is expressly pointed out
that the present invention is not limited to the use of two detectors.
Instead, the present invention may always include only one detector or
more than two detectors. In addition, the detector or detectors need not
be located in such a way that they are movable. Detectors 14 and 15 are
connected to an evaluation unit 26, using which integrating measurements
may be carried out in particular. For example, the electron beam is
guided to a raster point at which a measurement is performed repeatedly
using the electron beam, and the measured values obtained in this manner
are integrated in order to calculate the mean (pixel integration). The
integrating measurement may also be carried out for all raster points
located in one raster line (line integration). As an alternative, the
integrating measurement may also be carried out for several raster lines,
each of which includes raster points, and which are combined to form one
unit (frame integration).

[0050]Scanning electron microscope 1 also includes a particle current
measuring device 6, which is provided to measure the current of the
electron beam. In this exemplary embodiment, particle current measuring
device 6 is provided with a sensitive region 6'. Particle current
measuring device 6 is connected via a line 8 to an evaluation unit 7,
which, in turn, is connected via a line 9 to scanning means 4.

[0051]Scanning electron microscope 1 operates using the method according
to the present invention, as explained above with reference to the
exemplary embodiment depicted in FIG. 1. In this regard, reference is
made to the explanations provided above. Scanning electron microscope 1
also includes a change means 24, which is connected with beam source 2.
Once it has been determined that a change occurred in the current of the
electron beam (electron current), the electron current is changed to a
setpoint value. This ensures that the newly created images have no abrupt
changes in brightness, and that they are neither too light nor dark. Of
course, change means 24 may also be designed as shown in the embodiment
in FIG. 1.

[0052]FIGS. 3a through 3c show a schematic depiction of an object 5, on
which the raster image points are depicted schematically, the raster
image points having been scanned using the particle beam according to the
exemplary embodiment shown in FIG. 1, or using the electron beam
according to the exemplary embodiment shown in FIG. 2. Reference numeral
21 labels the various rows of raster image points on object 5. The
particle beam or electron beam 22 is guided over object 5 in such a way
that it approaches each raster image point in a row 21 in succession,
thereby "scanning" them. FIG. 3a shows the position of the particle beam
or electron beam 22 when a first raster image point in a row 21 is
scanned. FIG. 3b shows the position of the particle beam or electron beam
22 when a further raster image point in the same row is scanned.

[0053]If it is determined that the particle current or electron current 22
has changed, the location is determined at which the change occurred in
the particle current or electron current 22. This location is the raster
image point labeled with reference numeral 23 in FIG. 3b. After this
location is determined, the particle beam or electron beam 22 is guided
via the scanning means back to raster image point 23 (FIG. 3c). The
scanning of row 21 in which raster image point 23 is located is then
continued from raster image point 23 onward. All previous information
that was obtained by scanning from raster image point 23 to the raster
image point at which the scanning procedure was interrupted and the
particle beam or electron beam 22 was returned is discarded and replaced
with the information obtained in the repeated or new scanning procedure.

[0054]The same applies for the exemplary embodiment depicted in FIGS. 4a
through 4d. In this case, in contrast to the exemplary embodiment
depicted in FIGS. 3a through 3c, the scanning of the rows is continued
after it is determined that a change has occurred in the particle current
or the electron current (FIG. 4b), and the particle beam or electron beam
22 is returned to raster image point 23 only after entire row 21 has been
scanned (FIG. 4c).

[0055]As an alternative to returning to raster image point 23, it may be
provided in both exemplary embodiments for the particle beam or electron
beam 22 to not be guided to raster image point 23, but rather to the
beginning of row 21, in which raster image point 23 is located. This is
also depicted in FIGS. 3c and 4d. In this exemplary embodiment, the
entire row 21 is therefore rescanned. The present invention is therefore
not limited to returning the particle beam only to the raster image point
(raster image point 23 in this case) at which the change occurred in the
parameter of the particle beam (the particle beam current in this case).
Instead, the particle beam may also be guided to a further raster image
point that is located before the raster image point at which the change
occurred. The only essential point is that rescanning is also carried out
at the raster image point at which the change occurred in the parameter
of the particle beam current.

[0056]FIGS. 5a through 5c show an object that is being scanned in columns
25. The same descriptions that were provided for the exemplary
embodiments depicted in FIGS. 3a through 3c, and 4a through 4d, also
basically apply here.

[0057]The method described below may also be used, in particular, in the
exemplary embodiments shown. If it is determined via pixel integration
that a change occurred in the particle current or the electron current at
raster image point 23, the particle beam or electron beam 22 is returned
to raster image point 23. If line integration is carried out and it is
determined that the change in the particle current or the electron
current occurred at raster image point 23, the particle beam or electron
beam 22 is returned to raster image point 23 or a further raster point in
raster row 21 or raster column 25 that is located before raster image
point 23 (e.g., the first raster image point of raster row 21 or raster
column 25, i.e., the start of a raster row or a raster column). The same
applies for the frame integration, in which case it is also provided that
the particle beam is returned to raster image point 23 itself, or it is
returned to a further raster point in the frame located before raster
image point 23 (e.g., the beginning of one of the raster rows 21 or
raster columns 25). In all of the exemplary embodiments, scanning is
restarted after the return.

[0058]As mentioned above, once it is determined that a change has occurred
in the particle current or the electron current, the particle current or
electron current is changed to a setpoint value. This ensures that the
newly created images have no abrupt changes in brightness, and that they
are neither too light nor dark. This is an advantage in particular when
automatic adjustment of the image brightness is used. Good image quality
without any abrupt changes in brightness is always ensured via the method
described according to the present invention of returning the particle
beam and repeating the scanning procedure.